Engines may be configured with direct fuel injectors (DI) for injecting fuel directly into an engine cylinder and/or port fuel injectors (PFI) for injecting fuel into an intake port of an engine cylinder. Fuel injectors often have piece-to-piece and time-to-time variability due to imperfect manufacturing processes and/or injector aging, for example. Over time, injector performance may degrade (e.g., injector becomes clogged) which may further increase piece-to-piece injector variability. As a result, the actual amount of fuel injected to each cylinder of an engine may not be the desired amount and the difference between the actual and desired amounts may vary between injectors. Such discrepancies may lead to reduced fuel economy, increased tailpipe emissions, and an overall decrease in engine efficiency. Further, engines operating with a dual injector system, such as dual fuel or PFDI systems, may have even more fuel injectors (e.g., twice as many) resulting in greater possibility for degradation of engine performance due to injector degradation.
Diverse approaches may be used to estimate the variability in injector performance. One example approach is shown by Surnilla et al. in US20150159578 wherein direct injector variability is learned. A high pressure pump is operated to raise a direct injection fuel rail pressure, and then the pump is deactivated. Fuel is subsequently direct injected in a predetermined sequence for a predetermined number of times. Injector variability is learned by measuring a fuel rail pressure drop and an associated injector closing delay following each injection event. The pressure drop is corrected to account for the increase in closing delay, and then the corrected pressure drop is correlated with the amount of fuel delivered by the injector. By comparing the commanded fuel mass to the delivered fuel mass, an injector variability is learned.
The inventors herein have identified potential issues with the above approach. Specifically, the approach of Surnilla may not be able to reliably and non-intrusively diagnose a port injector. As one example, diagnosis of the port fuel injector would require the lift pump to be deactivated. However, disabling the lift pump could negatively impact the operation of the downstream high pressure pump, and thereby affect fueling of the cylinders via the direct injectors. As a result, the port injector may not be diagnosed non-intrusively. As another example, the measured pressure drop following a port injection event may be inaccurate at lower fuel rail (or port injection) pressures as well as at lower port injection volumes, such as may occur at low load conditions. Specifically, the fuel quantity injected as a “percent of value” may have reduced accuracy as the fuel quantity or pulse width commanded to the port injector decreases, resulting in inaccurate pressure drops being measured. Likewise, at lower fuel rail pressures, there is a possibility of fuel vapor being ingested instead of liquid fuel, resulting in inaccurate pressure drops being measured. As yet another example, the measured pressure drop may be affected by the voltage applied to the port injector. Inaccuracies in the pressure drop measurement may translate to inaccuracies in injector variability estimation. Injector offset results from the difference in injector opening time and injector closing time. If injector opening delay and closing delay were identical and otherwise symmetric, injector offset would be negligible. However, injector opening is governed by the supply voltage, injector resistance, and injection pressure (for a given injector design and fuel condition). Injector closing is governed by a distinct set parameters. Fuel injector errors can result in air-fuel ratio discrepancies in cylinders, leading to misfires, reduced fuel economy, increased tailpipe emissions, and an overall decrease in engine efficiency. The inventors herein have recognized that a port injection fuel rail pressure may be held elevated for a limited duration following suspension of lift pump operation. The fuel rail pressure may be further increased (e.g., above a fuel line pressure), while extending the duration of operation at the elevated pressure, by including a parallel pressure relief valve upstream of an inlet of the port injection fuel rail. The elevated pressure allows the pressure drop following an injection event to be amplified and learned more accurately. In addition, the port fuel injection may be more fuel vapor tolerant than expected. As a result, port fuel injection accuracy may increase when operated at or around the fuel vapor pressure with the lift pump disabled because the vapor pressure is substantially constant and free of fuel injection-caused pressure pulsations. At the same time, a high pressure fuel pump may be disabled and fuel pressure may be held in the high pressure fuel system by virtue of the fuel's bulk modulus.
By leveraging these factors, injector variability of a port injection system may be learned by a method for an engine comprising: port fueling an engine with fuel rail pressure above a threshold pressure and a lift pump disabled; learning variability between port injectors of the engine based on a measured drop in the fuel rail pressure, as a function of each of injection pressure and injection voltage, for each injection event of the port fueling; and adjusting subsequent port fueling of the engine based on the learning. In this way, variability between port injectors of an engine may be accurately learned and port fuel injector transfer functions may be updated accordingly.
As an example, responsive to port fuel injector calibration conditions being met, a lift pump may be operated to raise a port injection fuel rail pressure above a threshold pressure, and thereafter the pump may be disabled. Even after turning off the lift pump, the fuel rail pressure may be held at or above the fuel line pressure via a parallel pressure relief valve coupled to an inlet of the fuel rail, thereby accentuating a pressure drop at subsequent injection events. Port injector variability may then be learned by sweeping injection pressure while maintaining injection voltage initially at a first setting and then correlating fuel rail pressure drop at each port injection event to a parameter indicative of injector variability as a function of injection pressure. Next, injection voltage may be swept while maintaining injection pressure and then correlating fuel rail pressure drop at each port injection event to another parameter indicative of injector variability as a function of injection voltage, while the lift pump is disabled. A transfer function correlating fuel pulse-width to fuel mass may then be adjusted based on the learned parameters, thereby accounting for injector variability due to each of injection pressure and injection voltage. During subsequent port fuel injection, the updated transfer function may be applied.
In this way, by enabling a port injection fuel rail pressure to be held elevated above a fuel line pressure while a lift pump is disabled, it is possible to provide sufficiently large injection quantities to sustain an accurately measurable fuel rail pressure drop during port injector calibration. Additionally, fuel injection accuracy can be improved even at low injection volumes by maintaining fuel rail pressure within a threshold of the fuel vapor pressure. The technical effect of sweeping each of injection pressure and injection voltage with a lift pump off is that a port injector transfer function can be learned while accounting for variability due to both injector voltage and injection pressure. Further, the port injector variability can be learned by running at any fuel pulse-width, rendering the routine non-intrusive. Furthermore, by relying on the bulk modulus of fuel in a high pressure fuel system for maintaining pressure in the high pressure fuel rail, the port injector variability can be learned without disrupting direct injector operation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.